> For the complete MongoDB documentation index, see www.mongodb.com/docs/llms.txt

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# Default MongoDB Read Concerns/Write Concerns

## Read Concern

![Read/Write Concern Inheritance](/images/read-write-concern-inheritance.bakedsvg.svg)

### Default Read Concern

The default [read concern](https://www.mongodb.com/docs/manual/reference/read-concern.md#std-label-read-concern) is as follows:

| Operations | Default Read Concern |
| --- | --- |
| Reads against primary | [`"local"`](https://www.mongodb.com/docs/manual/reference/read-concern-local.md#mongodb-readconcern-readconcern.-local-) This read concern can return data that may be rolled back.; This read concern does not guarantee [causal consistency.](https://www.mongodb.com/docs/manual/core/read-isolation-consistency-recency.md#std-label-sessions) |
| Reads against secondaries. | [`"local"`](https://www.mongodb.com/docs/manual/reference/read-concern-local.md#mongodb-readconcern-readconcern.-local-) This read concern can return data that may be rolled back.; This read concern does not guarantee [causal consistency.](https://www.mongodb.com/docs/manual/core/read-isolation-consistency-recency.md#std-label-sessions) |

### Specify Read Concern: MongoDB Drivers

### Operations outside Transactions

The following information applies to operations that are run outside [transactions](https://www.mongodb.com/docs/manual/core/transactions.md#std-label-transactions). For read concern information related to operations that are run inside transactions, click the `Operations in Transactions` tab.

Using the [MongoDB drivers](https://www.mongodb.com/docs/drivers/), you can override the default [read concern](https://www.mongodb.com/docs/manual/reference/read-concern.md#std-label-read-concern) and set read concern for operations at the following levels:

| Level | Description |
| --- | --- |
| Client level | Applies to operations unless a finer-grained read concern for an operation is set at the database/collection/operation level. |
| Database level | Applies to operations on the database's collections (i.e. overrides the client read concern) unless a read concern has been set at the collection level or the operation level. Does not apply to operations inside transactions. |
| Collection level | Applies for read operations on the collection (i.e. overrides the database/client read concern) unless a read concern has been set at the operation level. Does not apply to operations inside transactions. |
| Operation level | Applies for the specific read operation (i.e. overrides the database/client/collection read concern). The ability to set read concern at the operation depends on the driver. Refer to your [driver's documentation.](https://www.mongodb.com/docs/drivers/) Does not apply to operations inside transactions. |

### Additional Information

For more information on the available read concerns, see [Read Concern.](https://www.mongodb.com/docs/manual/reference/read-concern.md)

## Write Concern

![Read/Write Concern Inheritance](/images/read-write-concern-inheritance.bakedsvg.svg)

### Default Write Concern

The implicit default [write concern](https://www.mongodb.com/docs/manual/reference/write-concern.md#std-label-write-concern) is [`w: majority`](https://www.mongodb.com/docs/manual/reference/write-concern.md#mongodb-writeconcern-writeconcern.-majority-). [`w: majority`](https://www.mongodb.com/docs/manual/reference/write-concern.md#mongodb-writeconcern-writeconcern.-majority-) ensures write durability by requiring replica sets to wait for on-disk journaling by default, controlled by [`writeConcernMajorityJournalDefault`](https://www.mongodb.com/docs/manual/reference/replica-configuration.md#mongodb-rsconf-rsconf.writeConcernMajorityJournalDefault). However, there is an edge case for replica set deployments containing [arbiters:](https://www.mongodb.com/docs/manual/core/replica-set-arbiter.md#std-label-replica-set-arbiter-configuration)

- The voting majority of a replica set is 1 plus half the number of voting members, rounded down. If the number of data-bearing voting members is not greater than the voting majority, the default write concern is `{ w: 1 }`.

- In all other scenarios, the default write concern is `{ w:
  "majority" }`.

Specifically, MongoDB uses the following formula to determine the default write concern:

```none
if [ (#arbiters > 0) AND (#non-arbiters <= majority(#voting-nodes)) ]
    defaultWriteConcern = { w: 1 }
else
    defaultWriteConcern = { w: "majority" }
```

For example, consider the following deployments and their respective default write concerns:

| Non-Arbiters | Arbiters | Voting Nodes | Majority of Voting Nodes | Implicit Default Write Concern |
| --- | --- | --- | --- | --- |
| 2 | 1 | 3 | 2 | `{ w: 1 }` |
| 4 | 1 | 5 | 3 | `{ w: "majority" }` |

- In the first example:

  - There are 2 non-arbiters and 1 arbiter for a total of 3 voting nodes.

  - The majority of voting nodes (1 plus half of 3, rounded down) is 2.

  - The number of non-arbiters (2) is equal to the majority of voting nodes (2), resulting in an implicit write concern of `{ w: 1 }`.

- In the second example:

  - There are 4 non-arbiters and 1 arbiter for a total of 5 voting nodes.

  - The majority of voting nodes (1 plus half of 5, rounded down) is 3.

  - The number of non-arbiters (4) is greater than the majority of voting nodes (3), resulting in an implicit write concern of `{ w: "majority" }`.

### Specify Write Concern: MongoDB Drivers

### Operations outside Transactions

The following information applies to operations that are run outside [transactions](https://www.mongodb.com/docs/manual/core/transactions.md#std-label-transactions). For read concern information related to operations that are run inside transactions, click the `Operations in Transactions` tab.

Using the [MongoDB drivers](https://www.mongodb.com/docs/drivers/), you can override the default [write concern](https://www.mongodb.com/docs/manual/reference/write-concern.md#std-label-write-concern) and set write concern for operations at the following levels:

| Level | Description |
| --- | --- |
| Client level | Applies to operations unless a finer-grained write concern for an operation is set at the operation/database/collection. |
| Database level | Applies to write operations on the database's collections (i.e. overrides the client write concern) unless a write concern has been set at the collection level or the operation level. Does not apply to operations inside transactions. |
| Collection level | Applies to write operations on the collection (i.e. overrides the database and client write concern) unless a write concern has been set at the operation level. Does not apply to operations inside transactions. |
| Operation level | Applies to the specific write operation. The ability to set write concern at the operation depends on the driver. Refer to your [driver's documentation.](https://www.mongodb.com/docs/drivers/) Does not apply to operations inside transactions. |

### Additional Information

For more information on the available write concerns, see [Write Concern.](https://www.mongodb.com/docs/manual/reference/write-concern.md)

## Causal Consistency Guarantees

With [causally consistent client sessions](https://www.mongodb.com/docs/manual/core/read-isolation-consistency-recency.md#std-label-sessions), the client sessions only guarantee causal consistency if:

- the associated read operations use [`"majority"`](https://www.mongodb.com/docs/manual/reference/read-concern-majority.md#mongodb-readconcern-readconcern.-majority-) read concern, and

- the associated write operations use [`"majority"`](https://www.mongodb.com/docs/manual/reference/write-concern.md#mongodb-writeconcern-writeconcern.-majority-) write concern.